Computed tomographic (CT) images are commonly used to visualize rock-pore systems. CTscans are 2-dimensional (2D) cross sections generated by an X-ray source that either rotates around the sample, or the sample rotates within the source beam. Bulk density is computed from X-ray attenuation coefficients, and serial sections are used to construct 3-dimensional (3D) images. Digital models are built from conventional, microCT, nanoCT, and synchrotron CTscans. Resolution, inversely related to sample size, is on the millimeter to micron to sub-micron scale, depending on the device used. Petrophysical calculations, such as porosity and permeability, are strongly influenced by segmentation of pixels into rock vs. pore. Segmentation is especially difficult if a fraction of the pores are smaller than the resolution of the CT acquisition system.